Diimine

This article is about the class of compounds having two imine groups. For compound having the molecular formula N2H2 and often called "diimine", see diazene.

Diimines are organic compounds containing two imine (RCH=NR') groups. The most popular derivatives are 1,2-diketones and 1,3-diimines. These compounds are used as ligands and as precursors to heterocycles. Diimines are prepared by condensation reactions: the dialdehydes or diketones is treated with an imine and water is eliminated. Similar methods are used to prepare Schiff bases and oximes.

1,2-Diimines

The 1,2-diketimine ligands, also called α-diimines, include dimethylglyoxime as well as oxidized derivatives of o-phenylenediamine. The steric properties of the substituents on nitrogen provide a means to control the axial coordination sites on a square planar complex. Large planar substituents such as mesityl tend to be orthogonal to the MN2 plane. In this way, the axial coordination sites on a square planar complex are shielded. Such steric control is not possible for complexes of the related to 2,2'-bipyridine, glyoximate, and 9,10-phenanthroline ligands.

A substituted 1,2-diimine ligand and an idealized metal complex.

An example is glyoxal-bis(mesitylimine), a yellow solid that is synthesized by condensation of 2,4,6-trimethylaniline and glyoxal.[1]

1,2-Diketimines, but not the 1,3-diketimines, are “non-innocent ligands”, akin to the dithiolenes.

1,3-Diimines

For example, acetylacetone (2,4-pentanedione) and a primary alkyl- or arylamine will react, typically in acidified ethanol, to form a diketimine. 1,3-Diketimines are often referred to as HNacNac, a modification of the abbreviation Hacac for the conjugate acid of acetylacetone. These species form anionic bidentate anionic ligands.

Uses

Substituted α-diimine ligands are useful in the preparation of so-called post-metallocene catalysts for the polymerization and copolymerization of ethylene and alkenes.[2]

Diimines are precursors to NHC ligands by condensation with formaldehyde.[1]

References

  1. 1 2 Elon A. Ison, Ana Ison "Synthesis of Well-Defined Copper N-Heterocyclic Carbene Complexes and Their Use as Catalysts for a “Click Reaction”: A Multistep Experiment That Emphasizes the Role of Catalysis in Green Chemistry" J. Chem. Educ., 2012, volume 89, pp 1575–1577. doi:10.1021/ed300243s
  2. ↑ Ittel, S. D.; Johnson, L. K.; Brookhart, M. (2000). "Late-Metal Catalysts for Ethylene Homo- and Copolymerization". Chemical Reviews 100: 1169–1203. doi:10.1021/cr9804644.
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